7.2 Semantic Actions in YACC
TBoS p 156-169
Semantic actions are attached to grammar rules by following each rule by a :=. This YACC definition receives a list of as and changes them to bs.
(0-) (defcc <as>
a <as> := [b | <as>];
a := [b];)
(fn <as>)
(1-) (compile (fn <as>) [a a a a a])
[b b b b b] |
The first rule says that any input of the form a <as> is to be translated into an output consisting of b consed to the translation of <as>.
The syntax of <as> requires that the input be a non-empty list composed of occurrences of a. So (compile (function <as>) [a a a]) gives [b b b].
The second rule is the base case.
As in Shen, round brackets signify function applications and square ones form lists. The following reformulation is an example:
(14-) (defcc <sent>
<np> <vp> := (question <np> <vp>);)
<sent>
(15-) (define question
NP VP -> (append [(protect Is) it true that] NP VP [?]))
question
(16-) (compile (fn <sent>) [the cat likes the dog])
[Is it true that the cat likes the dog ?] |
<!> and <e> are both reserved non-terminals. <e> always succeeds consuming none of the input and under semantic completion returns the empty list.
<!> always succeeds and consumes all of the input and under semantic completion returns that remaining input. |
